Email Presentation to Friend

Embed Code

NUCLEAR CHEMISTRY PowerPoint PPT Presentation

NUCLEAR CHEMISTRY. What will be discussed in this chapter?. fundamental particles of the atom Types of forces holding up the atom and its particles together Nuclear stability Natural radioactivity and types of radioactive decay Artificial radioactivity Nuclear energy Health hazards.

Copyright Complaint Adult Content Flag as Inappropriate

Download Presentation

NUCLEAR CHEMISTRY

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

After 1932, physicists viewed all matter as consisting of only three constituent particles: electrons, protons, and neutrons.

Beginning in 1945, many new particles were discovered in experiments involving high-energy collisions between known particles. These new particles are characteristically very unstable, and have a very short half-lives, ranging between 10-6 and 10-23 s. So far, more than 300 of these unstable, temporary particles have been catalogued.

The current theory of elementary particles of atoms, the standard model, claims that all matter is believed to be constructed from only two families of particles:

“QUARKS and LEPTONS”

LEPTONS

Leptons (from the Greek word ”leptos” meaning, small or light) are group of particles which participate in the weak interaction.

Included in this group are : “electrons (e), muons (μ), and taus (τ).”

LEPTONS

They interact only through weak and electromagnetic forces.

There are six types of leptons.

Each lepton has its own antiparticle.

A neutrino is associated with each lepton.

Have elementary structure which means that they don’t seem to break down into smaller units.

TYPES OF LEPTONS

The neutrino tau hasn’t been discovered yet but its presence is believed.

Electron is the lepton having the smallest mass.

QUARKS

The unusual property of quarks is that they have fractional electronic charges.

Associated with each quark is an antiquark of opposite charge.

There are six types of quarks : up (u), down (d), strange (s), charmed (c), top (t), and bottom (b).

TYPES OF QUARKS

Protons and neutrons are formed as a combination of different types of quarks.

PROTON-- two up [(+ 2/3) + (+ 2/3) ]

& one down quark (-1/3)

NEUTRON-- two down [(- 1/3) + (-1/3) ] & one up quark (+2/3)

THE FUNDAMENTAL FORCES IN NATURE

The strong nuclear forces

The electromagnetic forces

The weak nuclear forces

The gravitational force

The strength decreases downward.

THE STRONG NUCELAR FORCES

These are the strongest forces holding the quarks in protons and neutrons together.

They have the shortest range (10-15 m), meaning that particles must be extremely close before their effects are felt.

The quarks are considered to be held together by the color force. The strong force between nucleons may be considered to be a residual color force.

THE STRONG NUCELAR FORCES

A property of quarks labeled color is an essential part of the quark model.

It has nothing whatever to do with real color provides distinct quantum states.

THE STRONG NUCELAR FORCES

Analogous to the exchange of photons in the electromagnetic force between two charged particles, Gluons are the exchange particles for the color force between quarks.

The color force involves the exchange of gluons.

THE STRONG NUCELAR FORCES

WEAK NUCLEAR FORCES

It is responsible for the radioactive decay of subatomic particles.

Its strength is about 10-5 times the strong forces.

It’s a short range force.

If the nucleon number in a nucleus isn’t too much, the attraction forces between the nucleons ,the strong nuclear forces, counteract the repulsion forces, the weak nuclear forces, between the protons.

If the nucleon number is too much (Z >83), then the distance between the nucleons will be big and the weak nuclear forces will approach zero (because of the increase in the distance between the nucleons). Therefore, the electrical repulsion forces between the protons will be more effective making the nucleus unstable and the nucleus undergoes radioactive decays (e.g., beta decay).

Nuclear Stability

The stability of an atom is the balance of the repulsive and attractive forces within the nucleus.

Nuclear Stability

If the repulsive weak forces outweigh the attraction forces, the nucleus is unstable and undergoes radioactive reactions spontaneously. Such nuclei are called “radioactive nuclei.”

Think about it…

How might a higher number of neutrons change the balance between the repulsive and attractive forces in a nucleus?

How might a lower number of neutrons affect this same balance?

Nuclear Stability

Light nuclei are most stable if they contain an equal number of protons and neutrons, if N=Z.

Nuclear Stability

Heavy nuclei are more stable if the number of neutrons exceed the number of protons (Remember that , as the number of protons increases, the strength of the Coulomb (repulsion) forces increase, which tends to break the nucleus apart).

As a result, more neutrons are needed to keep the nucleus stable since neutrons experience only the attractive nuclear forces.

Nuclear Stability

Therefore, the ratio of n0 /p+ determines the stability of a nucleus.

For the stable nuclei, this ratio is close to “1.”

This ratio is “1” for the atoms with atomic number smaller than 20 (though this has exceptions for some isotopes).

As atomic number increases, stable atoms have ratios greater than one, can reach 1.5.

Nuclear Stability

Elements having more than 83 protons do not have stable nuclei. The isotopes of all of these atoms are radioactive.

Nuclear Stability

The shaded cluster is the “band of stability.”

The stable nuclei are present in the band of stability.

The solid line represents a neutron-to-proton ratio of 1:1.

Nuclear Stability

The nuclei which aren’t in the band of stability are radioactive and try to enter the band of stability as a result of some radioactive decays.

Nuclear Stability

Nuclei to the right of the band of stability don’t have enough neutrons to remain stable.

Nuclei to the left of the band have too many neutrons to remain stable.

n/p >1

n/p <1

Stable Nuclei

Nuclei above this belt have too many neutrons.

They tend to decay by emitting beta particles.

Stable Nuclei

Nuclei below the belt have too many protons.

They tend to become more stable by positron emission or electron capture.

Stable Nuclei

There are no stable nuclei with an atomic number greater than 83.

These nuclei tend to decay by alpha emission.

Chemical Reactions vs Nuclear Reactions

Radioactive nuclei are generally classified into two groups:

1) unstable nuclei found in nature, which give rise to what is called “natural radioactivity.”

2) nuclei produced in the laboratory through nuclear reactions, which exhibit “artificial radioactivity.”

NATURAL RADIOACTIVITY

In natural radioactivity, the unstable nuclei undergo the radioactive reactions spontaneously until they reach a stable configuration.

These transformations are accompanied by releases of energy.

These radiations are

alpha radiation(ışıma)

beta radiation

positron emission(yayma)

electron capture

238

92

234

90

4

2

U

Th

He2+

+



TYPES OF NATURAL RADIOACTIVEDECAYS

4

2

He2+

1. Alpha Decay(2p+,2no)

Nucleus emits an alpha particle—two protons and two neutrons

Alpha particle is a helium nucleus.

1. Alpha Decay

For an atom which emits rays;

p+ number decreases by two,

Atomic number decreases by two,

n0 number decreases by two,

Mass number decreases by four.

1. Alpha Decay

Properties of α- rays

They have a fogging effect on the photographic films.

Charge:α - particle carry positive charge. Its nuclear charge is +2.

Mass: Mass of each α – particle is 4 times that of a proton or H-atom.

Penetration power: α - rays have very small penetration power .They are stopped by a sheet of paper.

Effect of human body: α - rays can be stopped by the skin but very damaging to the skin due to ionization power.